Expandable endovascular prosthetic implants, such as stents and stent grafts, can be loaded into a catheter for delivery and deployment at a lesion site, such as an aneurysm or dissection within a patient's vascular system. The catheter is configured to retain the prosthetic implant in a delivery configuration during delivery to the lesion site. The catheter typically includes an inner cannula spaced from an outer sheath to define a prosthesis retaining region for receiving the prosthetic implant. The prosthetic implant is loaded onto the inner cannula along the prosthesis retaining region, with the outer sheath retaining the prosthetic implant in the delivery configuration. After the catheter is delivered to the lesion site, the prosthetic implant may be deployed with the catheter, for example, with retraction of the outer sheath relative to the inner cannula away from the prosthetic implant to allow for expansion thereof. Accurate placement of the prosthetic implant should sufficiently cover the target lesion site for endovascular treatments or procedures and the ends of the implant should be engaged with healthy tissue. Covering undesired locations with the ends of the implant, such as unhealthy vessels and/or branch vessels, due to inaccurate implant placement may cause unfavorable clinical consequences, such as branch vessel occlusion, aneurysm propagation, and/or restenosis.
Relative movement of the outer sheath during implant delivery or loading can cause axial compression and/or movement of prosthetic implant away from its desired location. The primary cause of such axial compression and/or movement is due to frictional interference or contact between the outer sheath and the prosthetic implant having a relatively low columnar strength that is expanded against the surface of the outer sheath. The frictional interference with the outer sheath can be greater than the columnar strength of the prosthetic implant, which permits deformation of the implant in the longitudinal direction, thereby collapsing the prosthetic implant in an accordion-like fashion.
Further, such axial compression and/or movement of the prosthetic implant increases the risk of a misplaced implant. That is, the misplaced prosthetic implant may not sufficiently cover the lesion site because such axial compression foreshortens the implant less than the length of the lesion site, such movement axially offsets the prosthetic implant from the lesion site, or both. For instance, axial compression of the prosthetic implant such as stent grafts with interval spacings or gaps between discrete stent segments is an increased concern to the end user. For example, during the deployment of a prosthetic implant with such interval spacing, the stent segments can converge closer to one another, also known as “bunching.” Another concern is an end portion of the prosthetic implant can be moved away relative to a distal tip of the catheter, also known as “gapping.” Further, the amount of axial compression can result in stent overlap, which expands the cross-section of the implant to a degree that increases the retraction or pushing forces of the sheath necessary for deployment. In some instances, the cross-section of the implant can be sufficient to prevent any relative movement of the sheath.
In addition, loading of the prosthetic implant and the inner cannula subassembly within the outer sheath during assembly can cause such axial compression and/or movement of the prosthetic implant before deployment. Pushing or pulling the prosthetic implant and the inner cannula subassembly relative to the outer sheath typically causes axial compression or bunching of the prosthetic implant. Loading techniques with conventional catheters can result in significant axial compression of the prosthetic implant of up to 20% of the actual length of the implant. For example, for a prosthetic implant such as a stent graft having a nominal length of about 144 mm, the amount of axial compression from loading can be about 15-25 mm. With this degree of axial compression, the stent graft may be viewed under fluoroscopy to be much shorter than the nominal length, e.g., at about 119-129 mm. Consequently, the clinician may be deceived as to the actual length or loaded location of the stent graft, which may inadvertently lead to inaccurate placement of the prosthetic implant relative to the lesion site.
Thus, there remains a need to facilitate loading and/or deployment of a prosthetic implant for accurate placement of the prosthetic implant. Further, there remains a need to inhibit axial compression and/or movement of the prosthetic implant during loading and/or deployment of the prosthetic implant. The need potentially becomes more significant as the strut thickness in stents and/or the graft wall thickness become increasingly smaller to reduce the overall delivery profile of the introducer and implant.